The invention relates to the field of hydraulic pumps for automatic transmissions; in particular, it pertains to a jet pump having a high-speed stream that draws fluid from a sump and pressurizes the inlet of a positive displacement pump.
Pressurized fluid for actuating the friction control elements of an automatic transmission for a motor vehicle is provided by a line pressure control valve. A positive displacement pump is used to supply fluid from a sump to the control system for actuation and lubrication of the transmission components. The line pressure control valve maintains line pressure within acceptable limits by releasing, through the control valve, excess fluid produced by the pump. However, a positive displacement pump requires a constant supply of fluid at a flow rate equal to the flow rate at the pump outlet. The volumetric flow rate of the pump increases in proportion to the speed of the pump, which is driven directly from the crankshaft of an internal combustion engine.
Unless the flow rate supplied to the pump inlet is sufficient to equal or exceed the flow rate at the pump outlet, fluid pressure at the inlet can approach one atmosphere of negative pressure, in extreme conditions. Low pressure at the pump inlet causes cavitation, a condition in which an air-fluid mixture is drawn into the pump inlet. As a result of vaporizing or boiling the fluid, the pump can be damaged when the cavitation bubbles collapse in the constriction pump chambers. Furthermore, cavitation produces noise and pressure fluctuations in the hydraulic system.
At high speed, positive displacement pumps for automatic transmission produce a larger flow rate than is required by the hydraulic system it supplies. A large flow rate of fluid must be supplied to the pump inlet to avoid cavitation and other harmful conditions, particularly under high-speed conditions. It is important also to avoid back pressure at the outlet of the line pressure control valve, which would increase the pump load and lead to inefficient operation, reduced fuel economy, and possible overpressurization of the friction control elements.
There is a need for a system to provide a reliable steady stream of pressurized fluid at the inlet of a positive displacement pump. Furthermore, because the available space for the pump and the control body is small, the system must be compact.